So-called radio identification systems (also known as transponder or RFID systems) are widely used these days. Such wireless identification systems are used for example to set up access controls for vehicles, buildings or areas and to check access authorizations. Particularly in the motor vehicle sector, such systems are usually used for access control and/or as immobilizers. In such systems, in order to check the authorization, the data memory of a transponder is read by a reader and compared with reference data in an evaluation unit in order to identify the transponder. Transmission of the (digital) data takes place in a wireless manner via high-frequency signals.
Like all identification systems which operate on the basis of digital data, radio identification systems are also at risk of so-called brute force attacks and/or dictionary attacks. One possibility for fending off these attacks or making them more difficult is disclosed in the document U.S. Pat. No. 6,658,328 B1. Said document provides for special encryption at least of some of the data in order to prevent dictionary attacks.
However, one fundamental risk in terms of security against misuse of radio identification systems consists in the lack of a physical presence check of the authorized key. A certain degree of security is provided by a transmission or reception range of the reader and/or of the transponder which is as short as possible. However, although the ranges in particular of passive transponders (so-called tags) are relatively short, this security of the system as a whole can be overcome very simply by so-called relay attacks.
In this case, a relay station is connected between the evaluation unit (vehicle) and the transponder (user), which relay station forwards the high-frequency signals (data) of the vehicle and of the transponder even over relatively large distances. Therefore, even a transponder of the vehicle proprietor who is far away from his vehicle can be read unnoticed. To this end, a first transceiver is placed in the direct vicinity of the vehicle and a second transceiver is placed in the direct vicinity of the owner or of the transponder, and thus an activation signal of the reader and also the response signals (data) of the transponder are transmitted over any distance (i.e. depending on the transmission power of the two transceivers). The vehicle proprietor will usually not be aware of such a relay attack.
This risk of misuse is moreover particularly high in those radio identification systems in which no additional manual activation of the transponder is required for radio identification. However, these radio identification systems, which are known as passive entry systems, are increasingly being used since they provide increased convenience for the user.
By way of example, in such passive entry systems for motor vehicles, the transponder in the user's key is activated in a wireless manner as the user approaches the vehicle, by triggering a high-frequency signal of the reader (which in turn is triggered for example by operating the door handle). Said transponder then transmits the data stored therein (including the so-called key) to the interrogating unit (the reader in the car). The evaluation unit in the vehicle then checks the key with its own data and opens the doors of the vehicle in the event of successful identification. If the key is not transmitted or an incorrect key is transmitted, access is denied.